If the Old Mass was used by Popes since
Gregory I, if Quo Primum of Pius V forbade and condemned changes, if
Pius XII in Mediator Dei forbade innovations like the altar facing
the people and the vernacular, how can traditional Catholics be
disobedient for wanting the Mass of the ages?

They are not, as I will explain.

First, many features of the Mass do not enjoy
the longevity you mention. Liturgical studies encouraged by the
Popes since the 1800s have shown that while the core elements of the
Roman Mass have not changed many of the lesser elements have. The
most authoritative account of this history is Fr. Josef Jungmann's
two volumes The Mass of the Roman Rite. Up until Trent the
Mass could differ between regions and dioceses, as local
adaptations were made to a basic Latin Rite. Rome did not, and
practically could not, exercise so firmly her supreme authority over
the liturgy.

The Reformation changed that, and the Holy See
reserved authority to herself. The question is not whether the
Church was or is bound to the liturgical form promulgated for
universal usage in the Roman Rite by Pope St. Pius V through Quo
Primum, she is not, but who has authority to make changes. This
is why Pope Pius XII, Vatican II and the 1983 Code of Canon Law
all state that any changes in the Liturgy must come from the Holy
See. The Popes, like Pius V in Quo Primum and Pius XII in Mediator
Dei, decry and forbid innovations,
changes by those without authority. This safeguards the essential
forms, and thus the validity, of the sacraments. The Supreme
authority in the Church, a Pope or a Council in union with the Pope,
always has the authority to make changes to ecclesiastical
discipline, such as the liturgical forms that cloak the essential
matter and form of a sacrament. Pius XII made changes, John XXIII
made changes, Vatican II proposed changes, and Pope Paul VI and Pope
John Paul II have made changes. All have understood that Popes do
not bind other Popes or Councils in such matters, otherwise the
Supreme authority would not be Supreme.

As for the altar and the vernacular, it is up
to the Supreme Magisterium to determine when, and where, and for
whom, they are opportune. Both options of altar and language have
been used in the Church. These are prudential disciplinary matters
not matters of faith, which are protected by the charism of
infallibility. However, even in such disciplinary matters the Church
cannot err so as to undermine the validity of the sacraments,
otherwise Christ' promise to be with His Church would fail.
They are prudential judgments about which popes and Catholics may be
of different opinions. Popes and Catholics are not bound to respect
your and my opinion on the matter. Catholics are bound to respect
that of Popes, however, as a practical matter of ecclesiastical
communion. That is what the doctrine of Papal Primacy teaches.

...all the faithful of Christ must believe
"that the Apostolic See and the Roman Pontiff hold primacy
over the whole world, and that the Pontiff of Rome himself is the
successor of the blessed Peter, the chief of the apostles, and is
the true vicar of Christ and head of the whole Church and faith,
and teacherof all
Christians; and that to him was handed down in blessed Peter, by
our Lord Jesus Christ, full power to feed, rule, and guide the
universal Church, just as is also contained in the records of the
ecumenical Councils and in the sacred canons.

... the faithful of whatever rite and
dignity, both as separate individuals and all together, are bound
by a duty of hierarchical submission and true obedience, not only
in things pertaining to faith and morals, but also in those which
pertain to the discipline and government of the Church spread over
the whole world, so that the Church of Christ, protected not only
by the Roman Pontiff, but by the unity of communion as well as of
the profession of the same faith is one flock under the one
highest shepherd. This is the doctrine of Catholic truth from
which no one can deviate and keep his faith and salvation..."
[Dogmatic Constitution on
the Church, Vatican Council I, 1870]

All Catholics should respect the teaching and
authority of the Church – whether manifested through a Council or
a Pope. As for preferring the traditional rites, since the Holy See
has shown its solicitude for traditional Catholics it can hardly be
a matter of disobedience to prefer the Tridentine
Mass. It would be theologically untenable to argue that in
and of itself there could be any grounds for claiming so.
Disobedience is a matter of a person's will knowingly rejecting
lawful authority. Where that is not present there is no
disobedience. In matters of the liturgy, the Holy See is that
authority, and its willingness to satisfy the aspirations of
traditional Catholics is clear.